Generic surveying instruments using the time-of-flight method are, for example, laser scanners, profilers, rotation lasers, LIDAR, laser trackers and recently also total stations, as are used in geodetic measurement tasks. The principle used herein substantially consists of emitting pulsed electromagnetic radiation, such as for example laser light, onto a target to be surveyed, and subsequently receiving the reflection that is returned by the target, wherein the distance to the target is determined on the basis of the time of flight of the pulses. Such pulse time-of-flight measuring systems are standard solutions nowadays in many different fields of application.
Different approaches are used to detect the scattered-back pulses. In what is known as the threshold value method, a light pulse is detected if the intensity of the incoming radiation exceeds a specific threshold value. The other approach that is relevant here is based on the temporally very precise sampling of the scattered-back pulse. An optical signal that is scattered back by a target object is captured by a detector. The electric signal generated by the detector is converted into a digital signal sequence using an analog-to-digital converter. This digital signal is subsequently processed further typically in real time. In a first step, the signal, often modulated as a pulse, is detected through special digital filters and subsequently its position within the signal sequence is determined. By using a large number of sampling sequences, it is possible to identify a useful signal even under unfavorable conditions, such that even larger distances or noisy background scenarios can be managed.
Known as prior art are sampler apparatuses that sample the time axis with much more than 10 GHz. In the case of special sampler apparatuses, sampling is carried out by sampling many identical pulses with additional phase shift, and in the process very fast sampling rates in the gigahertz range are virtually realized even with slower hardware components. EP 1 832 897 B1 also describes that it is possible with the same ultrafast sampling method to receive pulses or pulse sequences that are coded or modulated in amplitude, phase, polarization, wavelength and/or frequency. Sampling of non-identical, for example amplitude-modulated pulses in the high frequency range is likewise possible, even if large amounts of data arise in a very short period of time during evaluation.
One of the simplest types of modulation is the marking of the individual impulses or the pulse sequences per distance coding. This is used for example for the purposes of re-identifiability. This recognition is necessary if an ambiguity occurs which can be caused by different situations in the time-of-flight measurement of pulses. In principle, the non-uniqueness is created by there being more than one pulse or a pulse group between the surveying device and the target object. This problem is also described in detail in document EP 1 832 897 B1. The non-uniqueness can additionally also be made more difficult on account of the fact that the pulses returning to the surveying device mix with one another, that is to say return with a different sequence than that with which they were sent. The latter can occur whenever a plurality of pulses are “in flight” at the same time, or in other words: if the transmission pulse distance is shorter than double the measurement distance.
A so-called ambiguity distance is characterized by the longest distance at which the reflected signal still returns to the measurement device within a transmission period, wherein the signal can comprise one or more pulses, that is to say a pulse sequence or a burst. For the time-of-flight measurement as for the phase difference method, the phenomenon of ambiguity is known and is generally resolvable up to that distance corresponding to the longest period (ambiguity distance). For example, the periodicity of the transmission signal sequence is matched such that the distance to the object remains within the ambiguity distance. An object distance which is further away thus results in a lower frequency and measurement rate, if ambiguity is intended to be avoided. The longest period, however, can also be coded by the long periodicity contained implicitly in the transmission signal coding. The simplest example to be mentioned here is the modulation consisting of two high frequencies, which generate long periodic beats. It is not even necessary here for both frequencies to be emitted at the same time. Such coding methods, which can be altered in many variants, up to the spread spectrum method having a high frequency content, among others, have already been used for decades in phase difference methods. It is also known that when using high frequencies having long periodic beats, the measurement rate does not lower, but instead only the latency time until the distance information is captured gets longer. The minimum latency time here corresponds to the time of flight of light over the ambiguity distance.
Additionally, sudden distance jumps between the surveying device and the respective target object can result in so-called pulse wrapping, that is to say a change in the sequence of the reflected pulses as compared to the original sequence upon emission. In particular, this occurs in fast rotating laser scanners or profilers, if a pulse that is transmitted later nevertheless arrives at the receiver earlier, since it was transmitted onto a closer located target object and the pulse distance was not large enough to let the previously emitted pulse arrive first.
Another ambiguity occurs if a pulse or a pulse sequence strikes more than one target object, for example partially strikes a house edge and partially an area located behind the house. As a result, the signal is reflected more than just once from different distances, which complicates the measurement evaluation since it can no longer be determined uniquely from where the second echo originates.
Distance measurement apparatuses with pulse coding are therefore known from the prior art. In EP 1 832 897 B1, for example, a time-of-flight measurement apparatus is described in which five or more pulses are in flight between transmitter and receiver at the same time. As a result, for example a distance measurement apparatus positioned at an aircraft can measure at a very high pulse rate with many pulses in the air at the same time. The invention described, however, does not concern any specific modulation recognition of receive pulses and no method for resolving overlapping or interleaved pulses of multiple targets either.
U.S. Pat. No. 6,031,601 A also furthermore discloses that for distance measurement, a polychromatic or monochromatic light source is modulated using a pseudo-randomized number code generator. The light received by the target is decrypted according to the coding and the distance is calculated therefrom. This solution has the disadvantage that the generated RN coding sequences (random noise) have a large duty cycle, that is to say a large ratio of pulse duration to period duration. As a result, practically only small “breaks” between the signal pulses are present, which in the case of multiple targets leads to overlaps and to no simple separation of multiple targets being possible anymore in the time range of the receive signal representation.
US 2015/0070683 A1 discloses a distance measurement method per phase or interval modulation for scanning time-of-flight instruments. Herein, the intervals from pulse to pulse are coded and the evaluation of the receive pulses is carried out using a time that has elapsed since the transmission time of the last emitted pulse. This relative time of flight is then compared to the relative time of flight of the preceding pulse pair. The ambiguity resolution here is based on the comparison of the observed relative difference scheme with an expected scheme according to the generation by the special phase modulation. The relative distances are different for each interval, as a result of which the uniqueness can be resolved by way of searching prespecified tables (determined by this special modulation). The teaching from US 2015/0070683 A1, however, has the disadvantage that it works only for this one special type of signal modulation, and also that the pulse identification in the case of multiple targets is no longer unique—enormous data gaps would result in this respect in the true/false adjustment. In the document US 2015/0070683 A1, the disadvantage of the susceptible ambiguity resolution is partially resolved or alleviated using an additional phase shift from vertical scanline to vertical scanline.
WO 99/13356 describes an optoelectronic distance measuring device which has burst modulation. This apparatus is able to generate a sequence of optical pulses with adjustable burst lengths and adjustable phase shifts. As a result, for example, the signal-to-noise ratio is situationally optimized, and the time resolution of the receive signal can also be improved. A robust signal evaluation to resolve a uniqueness problem, however, is not disclosed.